Slow wave circuit and method of fabricating same

ABSTRACT

The slow wave circuit for a microwave tube is fabricated by slotting transversely through an alumina or beryllia ceramic body with the slots intersecting with the bottom of a longitudinally directed trough in one side of the body to form a comb-shaped dielectric support having a generally concave ladder-shaped spine joined to an array of vane-shaped rungs. A serpentine-shaped ribbon of metal is deposited on the concave ladder-shaped spine and two such ladder supported circuits are disposed in mutually opposed facing relation for interaction with an electron beam. In the composite circuit, the ribbon-shaped slow wave circuit element is everywhere supported, in intimate contact, by the ceramic support structure.

United States Patent [72] Inventors Louis R. Falce [54] SLOW WAVE CIRCUIT AND METHOD OF FABRICATING SAME 7 Claims, 5 Drawing Figs.

[52] [1.8. CI SIS/3.5, 29/600, 3l5/39.3,29/25.l7 [51] lnt.Cl H01j 25/34 [50] Field of Search 29/600,

[56] References Cited UNITED STATES PATENTS 3,370,256 2/1968 Baur et a1. 333/31 3,373,382 3/1968 Diamand 333 31 A 315/35 3,382,399 5/1968 Garoff Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. AnorneysStanley Z. Cole and Gerald L. Moore ABSTRACT: The slow wave circuit for a microwave tube is fabricated by slotting transversely through an alumina or beryllia ceramic body with the slots intersecting with the bottom of a longitudinally directed trough in one side of the body to form a comb-shaped dielectric support having a generally concave ladder-shaped spine joined to an array of vaneshaped rungs. A serpentine-shaped ribbon of metal is deposited on the concave ladder-shaped spine and two such ladder supported circuits are disposed in mutually opposed facing relation for interaction with an electron beam. in the composite circuit, the ribbon-shaped slow wave circuit element is everywhere supported/in intimate contact, by the ceramic support structure.

PATEN'IEI] OCT 5 AA 3 6 1 0 999 FIG. 5.

SLOT THROUGH DIELECTRIC SUPPORT MEMBER WITH AN ARRAY OF TRANSVERSE SLOTS INTERSECTING WITH A TROUGH IN THE SUPPORT TO DEFINE A COMB SHAPED'DIELECTRIC SUPPORT HAVING A LADDER SHAPED SPINE AND VANE SHAPED RUNCS DEPOSIT A SERPENTINE SHAPED CIRCUIT'ON THE LADDER SHAPED SPINE PORTION OF THE SUPPORT BRAZE RUNC PORTIONS OF THE SUPPORT TO A METALLIC SUPPORT MEMBER HAVING A COEFFICIENT OF EXPANSION MATCHING THAT OF THE DIELECTRIC MATERIAL BRAZE TWO MUTUALLY OPPOSED COMB AND CIRCUIT STRUCTURES INTO A BARREL IN MUTUALLY OPPOSED RELATION IN TRANSVERSE REGISTRATION vTO FORM A RING AND BAR CIRCUIT INVENTORS LOUIS R. FALCE ALLAN w. soon BY QWMJ m ATTORNEY SLOW WAVE CIRCUIT AND METHOD OF FABRICATING SAME 1 DESCRIPTION OFTHE PRIOR ART Heretofore, slow-wave circuits for microwave tubes have been proposed wherein a layer of metal was deposited in the concave trough of a ceramic support. The ceramic support was then transversely slotted in a pattern such that one half of 1 .ia'ring and bar circuit was formed in theconcave face of the rough. The slotting removed the dielectric material between adjacent turns of the. circuit such that the interaction impedance of the total resultant'circuit was not impaired. Two such slotted ceramic members were then, disposed one above the other in mutually opposed facing relation and the two halves were brazed together to form a composite'ring and bar circuit of the type wherein the circuit was everywhere supported in intimate contact from the ceramic support structure, whereby a relatively thin conductive ribboricould form the circuit element and whereby heat generated 'on the circuit by interception of the beam or the like could be readily carried away from the circuit via the intermediary of the ceramic support structure, such as beryllia or alumina. Such a circuit is disclosed in U.S. Pat. application 609,522 filed Jan. 16, 1967 and assigned to the same assignee as the present invention. I

The problem with this prior art circuit was that the two halves of the circuit had to be brazed together to form the composite ring and bar circuit. The brazingfalloy typically included copper and gold and therefore the maximum operating I ;-temperature of the circuit was limited by the vapor pressure of the brazing alloy in vacuum. Generally the circuit could not be operated at a temperature in excess'of 600 C. as vaporization of the brazing alloy would occur in'the vacuum of the tube which was around .10-7 Torr, such evaporated brazing alloy material being deposited upon the ceramic parts of the circuit producing e tcessive RF loss and tending to short out parts of the circuit. Therefore, it is desirableto produce a microwave circuit which is everywhere supported from a dielectric sup port structure and which does not require the use of brazing alloys such that a refractory material may be employed for the circuit and the circuits operating temperature may be sub; stantially increased.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved slow-wave circuit for microwave tubes and methods of fabricating same.

One feature of the present invention of a slow-wave circuit including an electrically conductive ribbon of generally serpentine-shape deposited on a concave ladder-shaped spine portion of the dielectric support body, whereby the entire serpentine-shaped conductive circuit member is supported from the face of the dielectric body.

Another feature of the present invention is the same as the preceding feature wherein the slow .wave circuit includes a second ladder shaped dielectric support structure having a serpentine-shaped circuit formed 'ona concave surface of a trough running longitudinally of thebody, and wherein'the two dielectric support bodies are disposed with their circuit portions in mutually opposed transverse registration to define a composite slow-wave circuit generally of the ring and bar or ring and loop configuration. 3

Another feature of the present invention is the same as any one or more of the preceding features wherein the serpentineshaped circuit is deposited on the concave face of the ladder shaped support member by depositing the metal through a mask onto the spine portion of the ladder shaped support.

Another feature of the present invention is the same as the first and second features wherein the circuit is deposited on Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OFTI-IE DRAWINGS FIG. 1 is a perspective view of a dielectric support member slotted to fonn a comb-shaped member having a laddershaped spine and vane sh'aped rungs, I

FIG. 2 is an enlarged fragmentary foreshortened view of a portion of the structure of FIG. I taken alongline 2-2 in the direction of the arrows and depictiii'g the serpentine-shaped circuit deposited on the ceramic support,

the concave face of the support member, the dielectric body. is slotted to form the'rungs andto remove the circuit metal I between adjacent rungs, and wherein the serpentine-shape of from the support through a mask.

FIG. 3 is a sectional view of the structure of FIG. 1 taken along line 3-3 and modified to show an'additional support member joined to the dielectric'structure,

FIG. 4 is a schematic perspective view, partly fragmentary, of a microwave tube employing the circuit of the present invention, and

FIG. 5 is a flow diagram, in block diagramform, depicting the steps in the method for fabricating slow wave circuit of the present invention.

DESCRIPTION or THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 5, the slow-wave circuit and the method of fabricating same will bedescribed; In the first step of the method, an elongated ceramic bar I. as of alumina or beryllia, having a semicylindrical concave trough 2 formed in one side thereof is transversely slotted with an array of slots 3 intersecting with the trough 2 to define a comb-shaped dielectric support structure having a ladder-Shaped spine por tion 4 and vane-shaped rungs 5. The concave face of the ladder has a layer 6 of a refractory metal, such as molybdenum-tungsten or rhodium, deposited thereon to a thickness preferably on the order of five skin depths at the operating frequency of the microwave tube in which the slow wave circuit is to be employed.

The transverse slots 3 also pass through the refractory metal layer 6 such that a concave ladder shaped ribbon of metal is everywhere supported from the dielectric support structure I. The metal layer fi is conveniently deposited in the trough along that side of the dielectric member by vapor deposition, evaporation in vacuum; or by sputter deposition.

In a second step of the method (see FIGS. 2 and 5), a serpentine-shaped circuit 7 is formed on the ladder-shaped spine portion of the support 1. In the case where the metallic layer 6 'has a generally ladder-shape, the serpentine-shaped circuit is I developed a mask will be formed masking off the serpentineshape for the circuit as indicated at 7, such mask having holes at 8 such that an acid etch applied over the removes the metal from alternate sides of the ladder to fon'n the serpentine-shaped circuit 7. The serpentine-shaped circuit' 7 is also known in the art as a meander line. After etching, the photo resist material is washed from the circuit leaving the serpentine-shaped cirduit-7 fully supported from the spine of the ladder-shaped dielectric support body I.

As an alternative to depositing a uniform layer 6 of metal over the entire trough, the metal may be left off of the dielectric support structure I during the slotting operation and then a photo resist material'may be coated onto the entire surface of the ladder-shaped face of the support. The photo resist layer is then exposed in a pattern such that when the pattern is developed a mask i's form'ed having a serpentine-shaped hole therein conforming to the serpentine-shape of the circuit 7 to be formed thereon. A refractory metal material is then deposited throughthe serpentine-shaped hole in the mask onto the concave ladder-shaped spine to form the serpentineshaped circuit 7' supported from the dielectric body I, as shown. This type of circuit is also known as a meander line cir- Cult.

In the next step of the method (see FIGS. 3 and the ends of the rungs 5 which are remote from the concave face 4 are metallized with a suitable metallizing composition, such as a molybdenum-manganese conventional metallizing brazing composition. The metallized rungs 5 are then brazed to a metallic support member 9 made of a material having a thermal coefficient of expansion substantially the same as that of the material of the dielectric support 1. In the case of alumina or beryllia ceramic, a suitable metallic material for the support 9 is a porous tungsten matrix having the pores infiltrated with copper, such matrix being comprised of 65 percent tungsten and 35 percent copper by volume. Such a material is commercially available and is marketed under the trademark Elkonite. A- conventional brazing alloy may be employed for brazing the rungs 5 to the support 9, such conventional brazing alloy need not have a melting point nearly so high as that of the refractory metal forming the circuit 7 since the joint between the rungs 5 and support 9 will operate, in use, at temperatures well below the maximum operating temperature for the circuit 7. Conventional copper-gold brazing alloys are suitable for the joint between the rungs 5 and support 9.

in the next step of the method (see FIGS. 4 and 5), a pair of the comb supported serpentine circuits 7 are brazed into a barrel structure 11 of a microwave tube which includes an electron gun assembly 12 at one end and a beam collector structure 13 at the other with the beam path 14 between the gun and collector passing axially through the composite slow wave circuit formed by the mutually opposed serpentineshaped circuit halves 7. The waves traveling on the composite slow wave circuit interact with the electron beam and, in the case of an amplifier, microwave energy to be amplified is fed onto the slow-wave circuit 7 in the conventional manner and extracted at the opposite end in the conventional manner. In the case of a backward wave oscillator one end of the circuit is terminated in a resistive load and microwave energy is extracted from the other end.

The two serpentine-shaped circuit halves 7 are disposed in mutually opposed transverse registration with similar portions of the circuit disposed opposite the opposed similar portion of the other circuit. The circuit halves 7 are spaced apart at their closest points, corresponding to the bar portions of a ring and bar circuit, by an amount which is preferably on the order of one-tenth of the diameter of the circuit'7. The barrel 1 1, in the case of solenoid focused tube, is conveniently made of copper and the brazing alloy employed to braze the metallic support members 9 to the barrel 11 should preferably have a melting point below the melting point of the brazing alloy employed to braze support member 9 to the rungs 5.' in the case of a periodic permanent magnet focused microwave tube, the barrel 11 is conveniently made of hollow cylindrical shape. Also, as an alternative, the rungs 5 may be brazed directly to the inside of i the barrel 1 l and the barrel may be made of a stack of alternate rings of different materials having an overall coefficient of thermal expansion matching that of the ceramic of the dielectric body 1. One of the materials for the ring members may be a magnetic material, such as iron, to facilitate periodic permanent magnet focusing. In this case, the rungs 5 need not be of rectangular cross section but may be made of semicircular cross section conforming to the curvature of the inside of the barrel. For the case where the barrel forms a part of the magnetic focusing structure and has a coefficient of expansion matching that of the dielectric support, the alternate rings may be made of iron and molybdenum and 48 percent of the barrel should be iron and 52 percent molybdenum by volume.

in the circuit 7 of the present invention, rhodium is the preferred circuit material, as rhodium when evaporated to form the metallic ribbon shaped circuit 7 has a relatively high electrical conductivity and has a relatively high melting temperature of 1960 C. Rhodium also has a vaporization temperature of -7 Torr of 1350" C. Thus, by use of a rhodium circuit 7 without the requirement for brazing, the circuit may forming be operated in the neighborhood of l000 C. which is substanimum permissible power level in the tube may be increased and C-band tubes constructed according to the method of the present invention have produced 5 kilowatts CW power output over an octave of bandwidth.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, slotting an elongated dielectric support body with an array of transverse slots extending only partially through the support body and intersecting with the bottom of a longitudinally directed trough in one side of the body to form a comb-shaped dielectric support structure having a generally concave ladder-shaped face comprising a continuous longitudinally directed surface at each edge of the trough, the longitudinal surfaces being interconnected by an array of transverse surfaces.

2. The method of claim 1 including the step of depositing a serpentine-shaped electrically conductive circuit on the concave ladder-shaped face of the dielectric support.

3. The method of claim 1 including a step of bonding the base of the support body to a metallic support member having a coefficient of thermal expansion substantially the same as that of the dielectric support.

4. In a microwave tube apparatus, slow-wave circuit means electromagnetic interaction with a stream of electrons, THE IMPROVEMENT WHEREIN, said slow-wave circuit means includes an electrically conductive ribbon including at least a portion of generally serpentine-shape forming at least a portion of a slow-wave circuit for electromagnetic interaction with an electron beam, said circuit having convolutions of the serpentine-shape generally lying in the surface of a wide face ofsaid metallic ribbon, a dielectric support body supporting the entire wide face of the slow wave circuit portion of said ribbon, said dielectric support body having a concave trough portion extending longitudinally of the beam path, an array of slots directed transversely to the beam path and extending only partially through said dielectric support body, such transverse slots intersecting with the bottom portion of said trough in said structure to form a generally concave ladder-shaped face comprising a continuous longitudinally directed surface at each edge of said concave trough, said longitudinal surfaces being interconnected by an array of transverse surfaces, and wherein said serpentine-shaped ribof said slow wave circuit is disposed overlaying bon portion said concave ladder-shaped face of said dielectric support structure with similarly shaped parts being disposed in transverse registration, and said metallic ribbon being bonded to and supported from said underlying concave ladder-shaped face of said dielectric support structure.

5. The apparatus of claim 4 wherein said metallic ribbon is made entirely of a refractory metal material having a melting point above l900 C.

6. The apparatus of claim 5 wherein said metallic ribbon is made of rhodium.

7. The apparatus of claim 4 wherein said slow-wave circuit means includes a second serpentine-shaped metallic ribbon bonded to a concave ladder-shaped face of a second dielectric support body, and wherein said first and second metallic ribbons are disposed in mutually opposed nonelectrically contacting relation facing each other in substantial transverse conforming registration such that similar parts of each ribbon are in transverse alignment with each other to form two halves of a composite slow-wave circuit. 

1. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, slotting an elongated dielectric support body with an array of transverse slots extending only partially through the support body and intersecting with the bottom of a longitudinally directed trough in one side of the body to form a comb-shaped dielectric support structure having a generally concave ladder-shaped face comprising a continuous longitudinally directed surface at each edge of the trough, the longitudinal surfaces being interconnected by an array of transverse surfaces.
 2. The method of claim 1 including the step of depositing a serpentine-shaped electrically conductive circuit on the concave ladder-shaped face of the dielectric support.
 3. The method of claim 1 including a step of bonding the base of the support body to a metallic support member having a coefficient of thermal expansion substantially the same as that of the dielectric support.
 4. In a microwave tube apparatus, slow-wave circuit means forming electromagnetic interaction with a stream of electrons, THE IMPROVEMENT WHEREIN, said slow-wave circuit means includes an electrically conductive ribbon including at least a portion of generally serpentine-shape forming at least a portion of a slow-wave ciRcuit for electromagnetic interaction with an electron beam, said circuit having convolutions of the serpentine-shape generally lying in the surface of a wide face of said metallic ribbon, a dielectric support body supporting the entire wide face of the slow wave circuit portion of said ribbon, said dielectric support body having a concave trough portion extending longitudinally of the beam path, an array of slots directed transversely to the beam path and extending only partially through said dielectric support body, such transverse slots intersecting with the bottom portion of said trough in said structure to form a generally concave ladder-shaped face comprising a continuous longitudinally directed surface at each edge of said concave trough, said longitudinal surfaces being interconnected by an array of transverse surfaces, and wherein said serpentine-shaped ribbon portion of said slow wave circuit is disposed overlaying said concave ladder-shaped face of said dielectric support structure with similarly shaped parts being disposed in transverse registration, and said metallic ribbon being bonded to and supported from said underlying concave ladder-shaped face of said dielectric support structure.
 5. The apparatus of claim 4 wherein said metallic ribbon is made entirely of a refractory metal material having a melting point above 1900* C.
 6. The apparatus of claim 5 wherein said metallic ribbon is made of rhodium.
 7. The apparatus of claim 4 wherein said slow-wave circuit means includes a second serpentine-shaped metallic ribbon bonded to a concave ladder-shaped face of a second dielectric support body, and wherein said first and second metallic ribbons are disposed in mutually opposed nonelectrically contacting relation facing each other in substantial transverse conforming registration such that similar parts of each ribbon are in transverse alignment with each other to form two halves of a composite slow-wave circuit. 